A wave soldering machine is used to solder components to a printed circuit board upon which they have been pre-mounted. Basic wave solder machines consist of an impeller mounted on the lower end of a rotatably driven pump shaft. The shaft and impeller are submerged in a body of molten solder, and rotated by means of a motorized pump operating above the surface of the solder. The impeller motion produces a wave at the surface of the solder bath. A circuit board is passed over the top of a solder wave to just skim its surface. During this passage, strong, conductive solder joints must be formed between the components and the board. Impurities in the solder may weaken the solder joints. A common impurity found in the wave soldering process is "dross".
Dross accumulates on the surface of the solder bath. The accumulation of the dross presents three problems: interference with the solder joints on printed circuit boards; seizure of the impellers; and hazards to operators when it is cleaned off a surface of the solder bath. The necessity of closing down the machine to clean the dross reduces the productivity of the manufacturing process. Hence, it would be desirable to eliminate dross from the wave soldering process.
It has been found that dross is produced in a wave soldering system when the frictional forces of mechanical devices rotating within the solder break down the solder into a dust. The rotatably driven shaft which rotates the impeller effectively grinds the solder into its component materials. It would be desirable to protect the solder from the force of the rotational shaft, and thereby preclude the production of dross.
U.S. Pat. No. 4,011,980 issued Mar. 15, 1977 to Dvorak et al. Dvorak discloses a wave soldering device comprising a support tube descending from an impeller motor to an impeller housing. A rotatable shaft carrying an impeller is secured within the support tube by two bearings, one located at the top of the rotatable shaft near the impeller motor, the second located at the bottom of the shaft near the impeller housing. A solder tube attaches to the impeller housing and extends upward to a reservoir. An opening at the base of the housing allows the impeller to draw solder from the base of the solder bath into the impeller housing and propel the solder up through the solder tube to the reservoir. A seal, located at the base of the support tube near the lower bearing, is included to keep solder from damaging the bearing and the shaft. A problem exists regarding effectively enclosing the shaft to eliminate the production of dross.